Two basic approaches to improving the rise and/or fall time of a video signal are filtering and "edge replacement". The most elementary filter enhancement method comprises using resistance/capacitance (RC) peaking filters to effectively differentiate high frequency components of the video signal. While such filters provide some improvement in rise time, they tend to accentuate high frequency noise and produce overshoots that vary as a function of the rate-of-change of the video signal with the fastest changing signals receiving the most enhancement. In addition to providing non-uniform response, the overshoots of such filters for fast video transitions near white level can result in video levels beyond peak white thus resulting in "spot blooming" if not compensated for.
A superior filter approach for transition enhancement is provided by peaking filters of the transversal kind. In such filters, video input and output signals from a delay line are subtracted from a video signal obtained a center tap of the delay line. The resultant "peaking" signal exhibits a pre-shoot and an over-shoot for video transitions in either sense. Advantageously, the "shoots" provided by such transversal filters exhibit amplitudes that are proportional to the overall change in signal level.
An example of a transversal peaking filter is described, for example, by Bingham in U.S. Pat. No. 4,041,531 entitled TELEVISION SIGNAL PROCESSING APPARATUS INCLUDING A TRANSVERSAL EQUALIZER which issued Aug. 9, 1977 and is incorporated herein by reference. Advantageously, the addition of pre-shoots and overshoots to a video signal has the effect on a dark to light transition of making the image darker just before the transition and lighter just after the transition. The visual effect is that transitions are more sharply defined in addition to having an improved rise time during the transition interval. A disadvantage, however, is that the transition interval does depend on the magnitude of the transition so that moderate level changes receive less improvement than higher amplitude level changes.
The problems with filtering for transition time improvement may be overcome by the known technique of "edge replacement". In the edge replacement approach one makes no attempt to alter the rate of change of the input video signal. What is done is to detect signal changes and "replace" the changing edge with a "new edge" having a faster rise (or fall) time. Advantageously, the resulting reconstructed signal has a transition time which does not depend either on the derivative of the input signal or on the overall magnitude of the change of the input signal. There follows two examples of "edge replacement" video transition systems.
A first example of an edge replacement video transition system is described by Hitchcock et al. in U.S. Pat. No. 4,758,891 entitled METHOD AND APPARATUS FOR IMPROVING THE RISE AND FALL TIME OF A VIDEO SIGNAL which issued Jul. 19, 1988. In an embodiment of the disclosed system, edges of an incoming video signal are sharpened by switching the output furnishing the sharpened signal in sequence first to receive a signal delayed by a greater amount, i.e., a signal which has not as yet undergone transition, and, secondly, to receive a signal delayed by a lesser amount, i.e., one that has already completed the transition. The rise time of the affected edge thus depends only on the time required to switch from one terminal to the next. The same switching sequence is carried out regardless of whether the transition is a negative going or positive going transition.
A second example of an edge replacement video transition system is described by Hartmeier in U.S. Pat. No. 4,777,385 entitled SIGNAL TRANSIENT IMPROVEMENT CIRCUIT which issued Oct. 11, 1988. An embodiment of the disclosed system includes a tapped delay line which provides successively delayed replicas of the signal to be processed. A multiplexer, responsive to a signal transition, sequentially couples delayed signal from the center tap and taps more distant from the input of the delay line to an output terminal to effectively hold the initial value of the transition for approximately half the transition period. The taps at the input end of the delay line up to and including the center tap are then sequentially coupled to the output terminal to effectively advance, in time, the end value of the signal transition. The signal transition is thereby reduced to a time equivalent to the switching interval between taps.